The chemical deterioration of a material, such as a metal is called corrosion. Most metals begin to corrode as soon as they are in contact with water or moisture in the air, acids, bases, salts, oils and other solid or liquid chemicals. Metals will also corrode when they are exposed to gases like acid vapors, formaldehyde gas, ammonia gas and sulfur gas. The best known case of corrosion that we know is the rusting of steel. The corrosion of metals like aluminum, tin, copper and zinc stops after a thin layer of metal oxide forms on the surface of the metal. Metal oxide film protects the metal from further contact with oxygen and the corrosion process stops. Other metals corrode under normal atmospheric conditions for example; copper, brass and bronze are protected from on-going corrosion by the formation of a green patina or film called verdigris which is made up of copper carbonate. Buildings with copper clad roofs and trim develop green patina which stops any further corrosion of the roof. The rate of corrosion can vary and is usually higher in industrialized areas that have high sulfur and nitrogen pollutants. These chemicals combine with moisture in the air to produce very corrosive acids. Metals can be protected from corrosion by coating the metals. Various coatings can be used including painting, electroplating with chromium or zinc. Plating a metal with zinc is called galvanizing. Coating steel with chromium and nickel produces stainless steel which is resistant to rusting. Some metals are coated with plastic ceramics or other rubber compounds to protect the metal from deterioration. Corrosion can show up in different forms. It can form a pit, a crack or spread across a wide surface area. Localized corrosion is where an intense attack of the material has taken place on its surface of the metal while other parts of the surface are corroding at a much lower rate. This is usually caused by a flaw in the metal or because of something in the environment. Pitting corrosion shows up as cavities or holes in a material. This type of corrosion is more dangerous because it is harder to detect. For example if a plane’s wing would have pitting corrosion and it was undetected, the wing could snap off under high pressure.